Painting cardboard can be tricky. It is easy to warp and wrinkle the cardboard. And this is even more challenging when it comes to silver and gold. This is because these paint colors are rather weak and don't take to cardboard very well. So, the best thing you can do, to make your cardboard look like actual silver or gold is to use spray paint.
I recommend either Rustoleum or Krylon. They both work very well.

Watch the video below

The template to make your own gold or silver ingot is right here (pdf) : Ingot Template